The UK has raised its terror threat assessment to the highest level following a bomb explosion on a rush-hour train in London.
The explosion, which left at least 29 passangers injured, was caused by the detonation of an improvised explosive device, say police.
Armed police stand guard outside Parsons Green station Photo: AFP
Prime Minister Theresa May said the threat was now critical, meaning an attack is expected imminently.
The blast, at Parsons Green station on a District Line train from Wimbledon, is being treated as terrorism.
Twenty-nine people have been treated in hospitals, mostly for burns, though at least eight have now been discharged.
So-called Islamic State says it carried out the attack. A hunt is under way for the person who placed the device.
The area around the station has been evacuated, with specialist officers there securing the remains of the improvised device and ensuring it is stable.
Prime Minister Theresa May condemned the "cowardly" attack, which she said had "intended to cause significant harm".
The country's terror threat level was raide to its highest level after a review immediately after the attack.
Mrs May said security analysts have concluded a further attack might be imminent.
Police said some 1000 armed officers would be seen across the country after military assistance was requested.
Mrs May said the military would be providing support to police and would replace them on guard duties at certain sites that are not accessible to the public.
These will include Buckingham Palace, Downing Street, embassies and the Palace of Westminster and will enable armed officers to carry out patrols, police said.
Police Scotland also said it would be increasing the number of armed officers on patrol, particularly at key locations and crowded places.
Hundreds of detectives and MI5 are investigating the attack, which took place at 8.20am local time on an eastbound train.
Assistant Commissioner Mark Rowley earlier refused to say whether anyone had been arrested.
Pictures taken of the train show a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor. The BBC understands the device had a timer.
BBC security correspondent Frank Gardner said the bomb appeared not to have gone off.
An image from social media shows a white container burning inside a London Underground tube carriage. Photo: AFP / FP Photo / Ho / @RRIGS
Had it worked as intended, it would have killed everyone around it and maimed everyone in the train carriage for life, he said.
US President Donald Trump has spoken to Prime Minister Theresa May to convey his sympathies for those injured in the terrorist attack, the White House has said.
President Trump pledged to continue close collaboration with the UK to stop attacks and combat extremism, the statement added.
A Downing Street spokesman said: "President Trump called the prime minister earlier today to offer his condolences over this morning's cowardly attack in London."
US President Donald Trump had earlier tweeted that the "sick and demented" people behind the attack had been in the sights of the Metropolitan Police, prompting Mrs May to say it was not helpful to "speculate" on an ongoing investigation.
Mr Rowley asked the public to remain "vigilant", but said they should "not be alarmed".
He said anyone who took pictures or videos at the scene could upload them to ukpoliceimageappeal.co.uk.
London Mayor Sadiq Khan appealed for calm, saying the city would "never be intimidated or defeated by terrorism".
Witnesses speak of 'panic'
Witnesses to the incident have described seeing at least one passenger with facial injuries, while others spoke of "panic" as alarmed passengers left the train at the station, which is above ground.
Anna Gorniak, who was in the same Tube carriage as the explosion, said: "I could see a fireball filling the carriage and coming our way. At that moment, I started to run.
"In my mind I was praying, I probably thought for a second, 'That's it, my life is over.'"
Passenger Peter Crowley said he was sitting in the carriage when the explosion happened.
He said his head was burned by a "really hot intense fireball above my head" and added: "There were people a lot worse than me."
An eyewitness said flames engulfed one carriage and raced along the train, forcing passengers to trample others as they rushed for an exit.
Witnesses described seeing at least one passenger with facial injuries.
People stand in the street by a Police car close to Parsons Green underground tube station. Photo: AFP
Alex Littlefield, 24, said: "I was walking around the corner to the Parsons Green Tube station, and I saw the raised platform with everyone running and looking upset.
"I saw police officers, fire brigade, and telling people to get back.
"I saw masses of people and armed police. There were lots of very, very distressed people. We've been pushed right back now."
Members of the emergency services work near Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage Photo: AFP
Media technology consultant Richard Aylmer-Hall, who was sitting on the "packed" District Line train, said he saw several people injured, having apparently been trampled as they tried to escape.
The 53-year-old said "suddenly there was panic, lots of people shouting, screaming, lots of screaming".
"I saw crying women, there was lots of shouting and screaming, there was a bit of a crush on the stairs going down to the streets," he said.
No change in MFAT advice
The Ministry of Foreign Affairs and Trade has not changed its advice for New Zealanders travelling to London in the wake of the explosion.
Currently the ministry is advising travellers there is some risk to travelling to the UK given terrorist attacks in March, May, and June.
- BBC / Reuters